1. Field of the Invention
This invention relates to an aluminum alloy locally having a composite portion, and is best applicable to a piston for internal combustion engines, made of aluminum alloy.
2. Description of the Related Art
To improve the mechanical strength of an aluminum alloy, having a composite portion, wherein the aluminum alloy is penetrated into a porous metal under a high pressure, is disclosed, for example, in Japanese Patent Publication No. 60-25619 and Japanese Laid Open Patent No. 62-131771.
Moreover, to improve the wear resistance of an aluminum alloy, a layer, which is a chemical compound of the aluminum alloy and a porous metal made of iron, nickel, copper or the like, is disclosed in Japanese Laid Patent 59-21393, Japanese Laid Open Patent No. 59-218341 and Japanese Laid Open Patent No. 59-212159.
Meanwhile, conventional production methods for the porous metal are classified roughly into three groups as shown in Table 1. Among various factors which influence the tensile strength and the wear resistance of an aluminum alloy having a composite portion, the porosity of a porous metal forming the composite portion increases the tensile strength of the interface, but decreases the wear resistance of the composite portion, as the porosity increases.
Thus, if a porous metal produced by a method of Group 1, is used, the porosity of which is comparatively small, and chromium is not added to the porous metal, a poor composite portion is often obtained due to small changes in the factors during a compounded process, and it becomes difficult to make use of the advantage that the wear resistance increases as the porosity decreases.
On the other hand, if porous metals produced by methods of Groups 2 and 3, are used the porosities of which are comparatively large, the poor composite portion is not formed, but it becomes difficult to improve a wear resistance of the aluminum alloy.
Incidentally, the applicants of this invention have found out that if chromium is cemented into the porous metal by diffusion, the wear resistance of the composite portion of the aluminum alloy is much improved, and on the basis of such fact, the prior application was filed. However, as the porosity of the porous metal becomes less than 40%, the tensile strength of the composite portion is hardly improved, and it probably depends upon the following reason.
As the porosity becomes less than 40%, the diameter of pores in the porous metal becomes small, and penetration distances to the center of the porous metal, where the molten aluminum alloy is to reach, become large, so that it is difficult for the melt to reach the center of the porous metal during the compounded process. Thus, it is necessary to raise the temperature of the melt or the preheating temperature of the porous metal, and as a result, the reaction of the aluminum alloy with the porous metal becomes active on the outer periphery of the porous metal by which a large amount of melt flows.